Selective stripping of electroplated metals



United States Patent Ofiice Patented Dec. 29, 1964 This invention relates to dissolving metals by chemical action and particularly it is concerned with selective solution of metals commonly encountered in electroplating operations.

In any electroplating plant a serious economic problem is centered around the recovery of those plated articles which have imperfections, discolorations, poor adhesion or other non-optimum characteristics of the plated film which make them unusable. The recovery of the base article by separating the plated metal from it is of prime importance in the electroplating field because of the value of the article being plated. Conventional methods of removing such faulty plates have included a variety of techniques. In our copending application filed July 1, 1957, Serial No. 668,858, now Patent 2,937,940, we have described an improved method of effecting recovery of plated articles.

The variety of prior art methods being practiced emphasizes the importance of the problem and, to some extent, the deficiencies of specific methods proposed. That is, none of the compositions in use in the art is sufiicient- 1y selective and safe for use in the removal of plated films from commonly used base metals. The strong acids are corrosive to practically every metal and the etched surface must be refinished after removal of the electro deposited plate. plated metal is also quite non-selective, because the voltages used cannot be made completely selective and because the electrolyte is inherently corrosive. The prime function of the current is to speed the reaction and the metal involved almost invariably forms a soluble salt which is dissolved in the electrolyte or which later precipitates as a sludge.

Chemical oxidation by strong oxidizing agents in the presence of caustic, cyanide or acid has serious drawbacks, because of the alkalinity, acidity and instability of the solutions. The base metals such as aluminum, zinc, lead, tin or other commonly used alloys are badly corroded by acid, caustic or cyanide. Fundamental in any of the existing operations also is the risk involved in using the very highly toxic cyanide in the nickel and copper plating shop, where most plating is done from acid solutions.

It is, accordingly, a fundamental object of this invention to provide a stripping solution for use in the reclamation of nickel plated objects, which has improved selectivity for removal of nickel plated metals while leaving the base metals relatively unaffected.

Other objects and advantages of the invention will in part be obvious and in part appear hereinafter.

In accordance with this invention, the improved selective stripping system is characterized by its combination of nitro organic compounds and ammonia and ammonium salts in water or water miscible solvents. More specifically the stripping medium is composed of a tertiary system of active components, which may be in a solvent, the first component being an aromatic nitro organic compound containing one, or more nitro groups, or mixture, or a n-itroaliphatic compound or mixture, wherein the aliphatic radical contains from one to about six carbon atoms, the second component being an acidic material capable of chemically neutralizing ammonia or Electrolytic oxidation of the ammonium hydroxide, preferably with the formation of a strongly buffered system; the third component being ammonia or ammonium hydroxide in a quantity sulficient to maintain a pH of about 7.5 to 11. (Acids that have been partially neutralized with metal alkalis are also useful such as NaI-ICO NaHSO NaI-I PO etc.) To

increase the water solubility of the nitro organic component in water it may carry other substituents such as hydroxyl groups, carboxylic groups or sulfonic acid' groups or an organic solvent-water system may be used.

Suitable nitro compounds are: o-, m-, p-nitrochlorobenzenes and mixtures thereof; 0-, m-, p-nitrobenzoic acids and mixtures thereof and nitrobenzene sulfonic acids and mixtures thereof; o-, m-, p-nitroanilines and mixtures thereof; o-, m-, p-nitrophenols and mixtures thereof. Other aromatic nitro compounds suitable for use in accordance with this invention may be found listed in United States Patent 2,698,781 and United States Patent 2,649,361.

The nitroparafiins are preferably those of relatively low molecular weight, which have some solubility in water and which are relatively non-volatile. Thus, the nitropropanes, nitrobutanes, nitropentanes, mono, di and trisubstituted, are useful.

Ammonia and ammonium compounds maybe used of either organic or inorganic acids, such as ammonium carbonate and ammonium bicarbonate; ammonium sulfate and ammonium bisulfate; ammonium phosphate, ammonium biphosphate, ammonium di-acid phosphate; ammonium nitrate; ammonium chloride. Ammonium salts of organic acids are also useful; for example, ammonium citrate, tartrate, oxalate, lactate, acetate, propionate, ben- Z'OMG, phthalate, maleate, etc, may be used; and also ammonium salts, of such amino acids a nitrilotriacetic acid, ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid salts.

Thus, the bath consists essentially of a solution containing an aromatic or aliphatic nitro compound, together with a buffer compound which buffers the pH in the range from about6-8, or the ammonium salt of an oxidation resistant acid plus sufiicient ammonia to raise the-pH to the range from about 7.5-11. The characteristic aspect of the system, therefore, is a high concentration of ammonium ions and undissociated ammonium compounds in solution, without the high pH which would be obtained if ammonium hydroxide were used alone to obtain the ammonia concentration needed for efficient operation.

It is fundamental to the process to maintain the solution basic and also so to regulate its composition to per- .mit operation at relatively high temperatures. The high temperatures accelerate the stripping action, but are inconsistent With the use of ammonia alone. Where operation at a temperature materiallyabove room temperature is contemplated, i.e., above about 35C., steps must be taken to fix the ammonia as a salt, or to provide for its replenishment as it is consumed or driven out of solution.

The stripping system may have a pH ranging from about 7.5 to a pH of about 11, but in no case, however, is a strong base used, such as caustic, to produce a high pH in the medium. The use of solutions at pH materially outside the limits indicated results in loweredefiiciency. In utilizing the material in accordance with this invention, the article may be dipped directly at ambient temperature of 20 -30 C. into a bath containing ingredients in the recommended proportions, with or without a solvent diluent, and agitated gently until the plated surface has been removed and base metal is uniformly exposed. The process may be speeded by raising the temperature of the bath to the boiling point but rapid NH, loss occurs even from the buffered system. If temperatures approximating boiling are desired, equipment may be pressurized; but an operating temperature of from 50 C. to 75 C. gives rapid stripping with little loss of NH from a properly manipulated system; i.e., simple covering of tanks is reasonably eifective.

The system has greater selectivity than conventional solutions, and we have found that pH control may be of substantial help in the stripping operation and in improving the effectiveness of the removal of plated material. Since the system contains substantial concentrations of ammonium ions with only relatively small concentration of free ammonia or hydroxide ions, metal ions are complexed without the use of excessively high p'Hs and we have found we can conduct stripping operations under conditions which cannot be handled by other systems.

For example, we have found that at pHs of approximately 8.5 the system has no real aflinity for the solution of copper, brass or zinc, but it will dissolve nickel rapidly. Even at the higher limits of pH zinc is dissolved slowly, but nickel is rapidly dissolved. This particular phenomenon is of great importance in stripping die castings since it is quite common to apply a flash coating of copper on a die cast article being plated and then to apply nickel over the copper. This selectivity is useful in removing nickel from a plated die cast article down to the copper flash, because salvaging the residual film speeds up the refinishing operation. However, in the absence of a copper film the nickel will be removed with no damaging corrosion of the die casting. (Zinc and zinc alloys are common die casting metals, as are aluminum and magnesium alloys.)

In preparing the stripping bath using the nitro acids identified, we have found the advantage of ammonia and ammonium salts resides not so much in their great stability or low volatility, but also in the fact that they have a butfer capacity and will perform as complexing agents over a wide range pH. T o a limited extent the strong mineral acids may be used to adjust or lower the pH of the system, but organic acids such as acetic, chloroacetic, lactic, citric, and the like, are satisfactory.

Through use of the stripping system to remove plated metals from the base the aromatic acid components gradually become spent and the ammonia gradually becomes more and more nearly saturated with metal ions being held in complex form. It is feasible, however, to make additions of both the aromatic nitro compound and the ammonia to a partially spent bath so as to maintain a useful concentration of the free ammonia and the nitro compound oxidizing agent. Nickel may be removed via cation exchange resin, precipitation or electrolysis and reclaimed.

The guide to the concentration of ingredients to be used is determined from the principles described. The concentration of the oxidizing agent may vary quite widely and is determined by the rate of solution needed for economic operation. It may be as low as 1 percent by weight in the solution, but preferably is about 540 percent. The concentration of the buffer compound may be equivalent on a molar basis with that of the oxidizing agent to saturation of the buffer compound in the solution. The amount of ammonia to be used is that needed to raise the pH to a level in the range from about 7.5 to about 11.

The invention is illustrated by the following specific examples.

A stripping bath of the following proportions of ingredients was prepared:

Mols Aromatic nitro compound 1 Ammonium oxalate Ammonia to pH 7-ll.

The molar ratio of the constituents may be varied widely, the optimum ratio being that which will exhaust the oxidizing agent which brings the metal being stripped into solution and the ammonia which complexes it at approximately the same time.

It is actually difiicult to balance proportions of ammonia and nitro compound in solutions so as to exhaust both simultaneously as is theoretically desirable. Hence, for operation we find that a solution carrying a substantial amount of ammonia radical is desirable. Since nickel in a complexed form unites with 6-12 ammonia groups, a useful and preferred ratio of ammonia in the solution is about 15 moles of ammonia radical per mole of nitro radical. In some instances it is also desirable to increase the excess level to a ratio as high as 25 moles of ammonia per mole of nitro compound. Here again the ratio is essentially based on the requirement of nickel consistent with chemistry and then a substantial excess beyond that requirement. Translated into weight units this can be stated in terms of about eight parts of nitrobenzoic acid to about 25 parts of ammonia. In the tabulation given below wherein typical ammonia systems are described in terms of 15 ingredients in 9 combinations of nitro compound, aqueous ammonia, and ammonia salt, formulations are given. These working concentrations illustrate working applications of these molar ratios.

The concentration of nitro compound-ammonia may vary widely. Effective removal of nickel is obtained with as little as 0.1 percent nitro compound in the presence of a saturated solution of ammonia-butter; or with as much as a saturated solution of nitro compound in the presence of the lowest concentration of ammonia-butter required to get the proper pH. The dilution of the active stripping composition may be as little as 1 percent of the nitro compound-ammonia mixture in 99 percent of water or other solvent. The effectiveness of the extreme dilutions or ratios is low, but they are operative.

Thus, in accordance with this basic operating principle, the following composition was tested:

EXAMPLE I Nitrobenzoic acid grarns 8.9 Ammonium sulfate .do 25 Ammonium hydroxide, aqueous (28 percent NH milliliters 60 Water do 200 More than 6 grams of nickel were stripped at C. There is, however, a rapid loss of ammonia at 80 C.

With a bath consisting of water 200 milliliters, 6.85 grams of nitrobenzoic acid, and sufiicient ammonia to maintain a pH of at least 10, we, found that we obtained good results, but without excessive loss of ammonia. This system is actually buffered with the NH in conjunction with the ammonium salt of the unreacted nitro compound. Additional buffer capacity, however, is desirable in order to maintain efficiency as the nitro acid ammonium salt is used up and reduced.

In place of the nitrobenzoic in the formulation given in the above example, the following nitro compounds may be used in the stated amounts:

(a) Di-nitropropane 3.55 grams. (b) Mixed isomers of nitrophthalic and isophthalic acids (commercial grade)- 11.48 grams. (c) Nitrobenzene sulfonate acid, crude mixture (sulfated, nitrated benzene)- 10.68 grams. (d) 0-, m-, p-nitro-phenols; 0-, m-, p-

nitro anilines 0.053 mol parts.

Generally any of the nitro organic compounds useful in cyanide stripping baths are useful in systems made in accordance with this invention with the advantage that pure single isomers need not be used. Thus, mixed 0-, m-, p-isomers of the compounds given in United States Patents 2,649,361 and 2,698,781 may be used.

EXAMPLE II because the ammonium salt is a buffering agent and the acid radical is a complexing agent.

We have found that nickel, if desired, can be easily reclaimed from the stripping bath. In some cases this can be done by merely adding sulfide ions, which precipitate nickel sulfide. Such recovery practices'are not practical when cyanide-based stripping baths are used. If desired the ammonia-nitro acid stripping bath may be acidified with concurrent release of metal ions which may then be plated from the solution by conventional electroplating procedures onto a sheet electrode. This nickel sheet can then be used in the conventional nickel bath thus effecting essentially 100 percent recovery of the metal stripped. The nickel can also be absorbed directly from the ammonia complex with'a cation ion exchange resin.

In general the stripping solution is used to reclaim a plated article, usually Ni, Co, Cd plate, because the Typical Ammonia Systems Nitrobenzoic acid, gm Sodium nitrobenzoate (98%), gm Sodium nitrobenzene sulionate (90%),

. Ammonium nitrate, gm..- Ammonium sulfamate, gm Ammonium citrate, gm Ammonium EDTA, gm

. Ammonium carbonate, gm Ammonium bicarbonate, gm Sodium bicarbonate, gm Sodium carbonate, gm

In the table, formulation 11 uses EDTA as an abbreviation for ethylene diamine tetraacetic acid. This is a typical amino polycarboxylic acid, useful as a buifer, because of its polycarboxylic character; other related acids are nitrilotriacetic acid, diethylenetriaminepentaacetic acid.

Ammonium carbonate and bicarbonate are preferred buffer agents, but are volatile and costly. Use of sodium bicarbonate with aqueous ammonia is eifectively as useful and avoids the need for attempting to store a costly volatile compound.

In the formulation of stripping solution in accordance with the table using the nitroaliphatic compounds, when molar ratios corresponding to those used for the nitroaromatics are employed, efficiencies, as measured by speed of stripping, are attained corresponding to about 70 percent of those reached with nitroaromatics. Useful nitroaliphatic compounds are nitromethane, nitroethane, nitropropane, hydroxynitropropane, trishydroxymethylnitromethane, nitrourea.

Observation of the solution may indicate that not all of the nitroaliphatic compound is in solution. As the stripping action progresses more of the nitro compound will go into solution, thereby to maintain a working concentration. Solvents may be used to put all components of the stripping composition in a single phase, but they do not necessarily improve efiiciency.

Based on a variety of observations involved in stripping metal from several bases using compositions made in accordance with the invention in which the nature of the ammonia source was varied or with other sequestering agents in the bath, we have determined that the value of the stability constant of the metal complex formed by the stripped metal and ammonia compound in solution seems not to be a determining factor in identifying whether a given complexing agent is workable. Where an ammonium salt of a complexing acid is used, such as ammonium salt of ethylenediaminetetraacetic acid an advantage in economy accrues, because the acid can be recovered from the spent bath by acidification. The solid acid form of ethylenediaminetetraacetic acid is precipitated, removed by filtration and reused. This and related acids are useful,

value of the article substantially exceeds the costof stripping. Hence, the solution will usually be in about the concentrations described in the examples. Extreme dilutions, of the order of 0.01 to 1 percent of active components are useful when the mass of the article is small. For example, if it is necessary to strip nickel from an extremely fine gauge copper wire, the solution described in Example I, at neutral pH, and concentration of about 0.01 percent is useful. It removes the nickel rapidly enough to be economically useful, yet the conditions are such that the copper base, itself very small, is'not affected.

Though the invention has been described in terms of a limited number of examples, it is to be understood that variants thereof may be practiced without departing from its spirit or scope.

What is claimed is:

1. A composition for selectively dissolving nickel, cobalt, cadmium and alloys thereof deposited on a basis metal, which composition consists essentially of an ingredient selected from each of the following groups, and 7 (a) nitro-substituted aromatic compounds containing at leastone N0 attached to a benzene ring, which benzene ring also carries a radical having a watersolubilizing effect on the nitro-substituted aromatic compound, and water-soluble nitro-substitu-ted ali phatic compounds and those which have a water solubilizing group attached to the aliphatic chain, and (b) water soluble ammonium salt,

the said composition being in the liquid phase, having a containing at least 0.01 percent by weight of the combination of nitro-containing compound and ammonium salt, in a ratio of about 1 mol of nitro-radical to about 15 mols of ammonium salt.

3. A composition which comprises, in liquid phase, about 8 parts of nitrobenzoic acid, about 25 parts of an alkali metal bicarbonate and about 25 parts of ammonia, said composition having a pH of about 7-11, containing at least 0.01 percent by weight of the combination of nitro-containing compound and ammonium salt, in a ratio of about 1 mol of nitro-radical to about 15 mols of ammonium salt.

4. A composition which comprises, in liquid phase, nitrobenzene sulfonic acid, acetic acid, and ammonia, in the pH range of about 7.5 to 11, said composition containing at least 0.01 percent by weight of the combination of nitro-containing compound and ammonium salt, in a ratio of about 1 mol of nitro-radical to about 15 mols of ammonium salt.

5. A composition which comprises, in liquid phase, a nitroisophthalic acid, aqueous ammonia and an alkali metal bicarbonate, said composition having a pH of about 7-11, containing at least 0.01 percent by weight of the combination of nitro-containing compound and ammonium salt, in a ratio of about 1 mol of nitro-radical to about 15 mols of ammonium salt.

6. The method of stripping electroplated nickel, cobalt, cadmium and alloys thereof from basis metal articles comprising providing an aqueous bath consisting essentially of a composition formulated with an ingredient from each of the following groups, (a) and (b);

(a) nitro-substituted aromatic compounds containing at least one N0 attached to a benzene ring, which benzene ring also carries a radical having a watersolubilizing effect on the nitro-substituted aliphatic compounds and those which have a water solubilizing group attached to the aliphatic chain, and

8 (b) an ammonium salt, the said composition being in the liquid phase having a pH about 7-11, containing at least 0.01 percent by weight of the combination of nitro-containing compound and ammonium compound, in a ratio of about 1 mol of nitroradical to about 15 mols of ammonium compound, and immersing said electroplated basis metal article in said bath,

maintaining said bath at a temperature in the range from about room temperature to boiling, and maintaining the pH in the range from about 7.5-l1, maintaining said article in said bath until said electroplated metal has been dissolved, and recovering from the bath said basis metal article substantially unaffected by said bath, 7 said bath being comprised of about 5 to 25' mols of ammonium ion per mol of nitro-radical in the nitro compound.

7. The method in accordance with claim 6 in which the bath is composed of nitrobenzoic acid and ammonium sulfate.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Pauling: General Chemistry, 2nd ed., 1954, W. H. Freeman & Co., San Francisco, page 5 84.

Karrer: Organic Chemistry, 1938, Elsevier, Amsterdam, page 112. 

1. A COMPOSITION FOR SELECTIVELY DISSOLVING NICKEL, COBALT, CADMIUM AND ALLOYS THEREOF DEPOSITED ON A BASIS METAL, WHICH COMPOSITION CONSISTS ESSENTIALLY OF AN INGREDIENT SELECTED FROM EACH OF THE FOLLOWING GROUPS, (A) AND (B); (A) NITRO-SUBSTITUTED AROMATIC COMPOUNDS CONTAINING AT LEAST ONE NO2 ATTACHED TO A BENZENE RING, WHICH BENZENE RING ALSO CARRIES A RADICAL HAVING A WATERSOLUBILIZING EFFECT ON THE NITRO-SUBSTITUTED AROMATIC COMPOUND, AND WATER-SOLUBLE NITRO-SUBSTITUTED ALIPHATIC COMPOUNDS AND THOSE WHICH HAVE A WATER SOLUBILIZING GROUP ATTACHED TO THE ALIPHATIC CHAIN, AND (B) WATER SOLUBLE AMMONIUM SALT, THE SAID COMPOSITION BEING IN THE LIQUID PHASE, HAVING A PH OF ABOUT 7-11, CONTAINING AT LEAST 0.01 PERCENT BY WEIGHT OF THE COMBINATION OF NITRO-CONTAINING COMPOUND AND AMMONIUM SALT, IN A RATIO OF ABOUT 1 MOL OF NITRORADICAL TO ABOUT 15 MOLS OF AMMONIUM SALT, IN WHICH THE AMMONIUM SALT IS OF AN ORGANIC ACID SELECTED FROM THE GROUP CONSISTING OF NITRILOTRIACETIC ACID, ETHYLENEDIAMINETETRAACETIC ACID, AND DIETHYLENETRIAMINEPENTAACETIC ACID. 